This invention relates to speed control of step motors, and in particular, to a closed loop speed control arrangement for step motors used to drive print heads of printing devices.
In the art, open loop driving methods have been used for step motors wherein no detector is provided for detecting the position or rate of rotation of the step motor. The "phase change" timing (as used herein, the term "phase change" refers to a change in the coil or coils of the step motor to which driving current is applied to effect driving) required for acceleration of the motor from a stopped position, deceleration of the motor from a moving state or constant speed operation, is selected in advance by calculation from a control curve experimentally determined and stored in a memory. Thus, by way of example, where a microprocessor or micro-computer controlled circuit is provided for driving the step motor, the abovementioned data for speed control is stored in a ROM. At the time that the driving of the step motor is to start, the first pre-determined stored time period is read from the memory and at the time that the first stored time period comes to an end, phase change (driving) of the motor is performed and, simultaneously, the next time period is read from the memory. This procedure is followed repetitively with the interval between successive phase changes following the program of stored time periods. In this way, the motor speed is raised to a constant speed.
The driving force f.sub.sT required to rotate a step motor for one step may be defined as follows: ##EQU1## where x represents the amount of rotation, m is the mass of the load, t.sub.sT is the time necessary to rotate the motor for the amount x, and fu is the kinetic friction force of the load.
Integrating equation (1) above, the following equation is obtained: ##EQU2##
During a constant speed period, the time period of the phase change required for constant speed rotation is set. Each time that the time period comes to an end, the phase is changed. The above action is repeated to drive the motor at a constant speed. During deceleration, as noted above, the calculated time period between phase changes is stored in memory. As the motor enters the deceleration condition, the first calculated time period is read out of the memory. At the end of that first time period, phase change is effected and the next time period is read out from the memory. This procedure is repeated until the motor slows and finally stops.
In such open loop control, where the motor is to be advanced only a slight incremental angle, half of a predetermined number of steps or phase changes are used for acceleration control and the remaining steps are used for deceleration control.
Accordingly, the prior art open loop step motor driving arrangements are characterized by calculated intervals of phase change based on anticipated acceleration and deceleration characteristics of the step motor. However, this prior art step motor speed control approach is characterized by certain disadvantages. First, this approach does not prevent vibration during acceleration or deceleration caused by variations in the power supply or variations of the load on the stepping motor. FIG. 1 illustrates this vibration, showing a curve wherein the ordinate represents the step distance (incremental advance) of the step motor while the abscissa represents the time elapsed. Times T.sub.a through T.sub.a+9 each represent one of the predetermined time periods calculated in advance for acceleration or deceleration control and stored in the memory.
Still another disadvantage of the prior art speed control approach is that the motor's speed cannot respond to driving pulses of the predetermined time period when the driving system of the step motor is locked for a period of time. This results from the fact that the interval between phase changes (driving) of the step motor is fixed by the pre-calculated time periods and does not take into account intervals where the driving system of the step motor is locked.
In U.S. Pat. No. 3,863,118, a closed-loop speed control system for step motors is taught wherein, after initial driving by an external pulse, the motor is driven by feedback pulses from a transducer connected to the motor output, the transducer being in the form of an optical position detecting device. An adjustable time delay is provided in the feedback loop for adjusting the effective switching angle or phase change time interval, the interval being in turn controled by a comparator, which compares an external timing signal with the feedback signal. By providing an apparatus for detecting and storing the time intervals between the phase changes during acceleration and using the same time intervals in reverse order to control phase changes during deceleration and/or by controlling the application of the phase change signal in response to the later of the end of a predetermined time period and a timing pulse representative of an incremental advance of the step motor, the foregoing disadvantages of the prior art are overcome and an improved speed control for step motors is provided. Further, by providing a driving circuit for the step motor having a diode for checking inverse current inserted between the excited coil and the excited current switching circuit provided for controlling the supply of current to the excited coil of each phase, where said drive circuit includes a spike-suppressor circuit, an improved step motor results.